Solar cell more efficient thanks to gold electrode

Electrons in a golden swing make water splitting more efficient.

I don't think it's too far from the truth to say that humanity is facing a number of simultaneous environmental crises, any one of which, if not addressed, could lead to dramatic changes to the quality of life for millions of people. One of those crises is energy. We can continue to use coal, gas, and oil for quite some time, but the cost is huge. The price of extraction is going up, the amount to be extracted is going down, and the emitted CO2 is dramatically changing our climate. With all of these downsides, an alternative is called for. One such alternative is the generation of fuels via solar energy, where we either burn hydrogen directly, or reform carbon dioxide and hydrogen into hydrocarbons to form a closed carbon cycle. If sufficient scale could be reached, solar fuels could be used to meet transport energy requirements in the future.

But for this to work, we need to make a number of processes more efficient on a per-molecule basis. One of those is more efficient water-splitting. The problem actually lies with the oxygen side of the reaction. When we split water each molecule produces a single, highly reactive oxygen atom. This atom must combine with another oxygen atom to create molecular oxygen. Making that happen efficiently requires a good catalyst, and four charges with rather high energy. Recent work published in Nano Letters shows how this might be achieved.

Haven't we being splitting water for ages?

For a long time the big problem with water splitting had been that oxygen, even molecular oxygen, is highly reactive, so it tended to oxidize the catalyst, rendering it useless in a short time. A couple of years ago, however, a new class of catalysts based on cobalt were developed. These catalysts appear to be long-lasting and efficient. With this development, some of the focus has turned to the high-energy charges. Typically, the energy possessed by an electron is something close to the amount of energy used to free it up. In this case, where the excitation is due to light, that means that only photons from the ultraviolet part of the spectrum have sufficient energy to partake in the water splitting reaction.

Luckily for us, the Earth's atmosphere does a pretty good job of absorbing ultraviolet light, but that means that solar water-splitting seems doomed to low efficiency simply because the number of photons is so small. There is, however, an alternative: surface plasmon resonances. A surface plasmon is generated through electrons moving back and forth in concert with an exciting light field. The point is that the energy stored in a plasmon can be increased simply by turning up the light intensity. The light drives the electrons to move further and faster, just as pushing on a swing drives it to ever larger amplitude swings and faster speeds—more precisely, the speed of the swing is faster when it passes through its rest position. Even though the individual pushes have insufficient energy to generate such a large motion, the swing stores and combines that energy with those from earlier pushes.

Surface plasmons being able to store that energy is all well and good, but if you set yourself up to use that energy, then you usually damp out the swinging motion. Think of it like a shock absorber: the spring excites a bouncy motion every time you hit a bump. Given the chance, the spring will keep the car bouncing along, giving you motion sickness and reducing your control of the vehicle. The shock absorber provides resistance to the bouncing, extracting the energy stored in the spring and turning it into heat. That is very useful in the case of a car, but, for surface plasmon resonances, it tells you that if you extract the energy from the electrons efficiently, your plasmon is going to vanish. In other words: no high energy charges will be available for water splitting.

A group of researchers from the University of California Santa Barbara have shown how to extract just the right amount of energy from the surface plasmons, so that efficient water splitting can occur. The surface plasmons are excited by light illuminating an array of tiny golden rods—the rods are 90nm in diameter, and around 200nm long. The rods are isolated at the bottom and top by a thin layer of insulating material (titanium oxide). The cobalt-based catalyst was attached to the gold nanorods around their middle. All of this was placed on a transparent electrode, which was connected to a platinum electrode, where hydrogen is evolved.

So how does this work? The gold nanorod limits the motion of the plasmon and the plasmon reflects off the end surfaces and meets itself coming the other way. The resulting interference pattern generates a huge electric field near the ends of the nanorod. Physically, the electrons tend to repeatedly pile up at the ends of the nanorods and then spread along the length of the rod. The insulating cap prevents most of the electrons from escaping, unless they have sufficient energy to efficiently tunnel through the insulating barrier. And this is precisely what happens. The electrons tunnel through the oxide layer into the electrode, over to the hydrogen evolving electrode, where they provide the electrons necessary to produce molecular hydrogen.

The loss of these electrons creates high-energy holes in the gold nanoparticle—a hole is the absence of an electron that behaves just like a positively charged particle—which get sucked up by the catalyst and given to the surface oxygen. The oxygen then comes off the electrode as a gas.

The clever part of this strategy is the tunnel barrier between the plasmon and the electrode. By creating this, the researchers can ensure that only the highest energy electrons have a significant chance of making it to the electrode. This sucks some energy out of the plasmon, but does not damp it out completely, allowing the light to replenish the plasmon's energy. The plasmons themselves are most efficiently excited by light in the visible part of the spectrum, as well as wavelengths a little bit longer than that. This just happens to be where most of the light in the solar spectrum is—measured after passing through the Earth's atmosphere. A single photon cannot generate a single electron with the requisite energy, and the plasmon has some natural damping, so it takes at least two, and more likely three or four photons to generate a single electron, depending on the photon wavelength. Nevertheless, that's still better than having direct excitation by ultraviolet light.

Am I going to get a gold solar cell now?

The results are an interesting mix of nice and confusing. Most—around 80 percent—of the generated electrons contribute to the water splitting reaction. They show that with ultraviolet light only, the current density drops by a factor of five to ten. They also show that without the cobalt catalyst, the reaction rate is much poorer. This is in spite of the fact that the insulating material is also a catalyst (TiO2): it is ineffective compared to the cobalt catalyst. The general picture is that it is the catalyst and the plasmons that contribute to the increased reaction rate, rather than any particular component.

There is, however, a very confusing result. In these experiments, the general procedure is to use a standard lamp—this is a lamp that, to some extent, mimics the solar spectrum. More importantly, it allows different solar devices to be compared, and to switch it on and off periodically. The changes between on and off parts of the cycle, combined with changes between different cycles, allow researchers to understand how the cell is performing. The researchers note that during the on-time, the current jumps to some high level, and then, instead of plateauing, it slowly increases by about a third. There is no explanation of this slower process.

To give you an idea of why this is confusing: the electron processes that would allow the plasmon to reach equilibrium occur on the scale of picoseconds. Their interaction with the crystalline gold atomic cores should have stabilized in a few nanoseconds. The water and electrolytes take a bit more time to respond—call that a few seconds. After that, everything should be stable, so what is going on here? The researchers offer no comment at all.

You might say that thermal processes mean that you are simply heating the cell up. This may very well be true. But for the visible and near infrared part of the spectrum, the processes described above are the microscopic description of thermal processes. That only leaves direct infrared absorption. It seems to me that they have filtered out the longer infrared contribution from the lamp, so where did the heat come from? This requires more explanation.

What next? Get rid of the gold. Gold makes an excellent carrier for surface plasmons, but there isn't a lot of it around. This needs to be replaced with a more common metal, like aluminum or copper. If they can do that without a substantial decrease in current density, then they will have a winner. It would also help if the hydrogen was in a more useful form—hydrogen gas is hard to store efficiently—like a hydrocarbon. That will involve splitting CO2, which is much more difficult.

I always wondered why the huge amount of energy generated in windmills that goes to waste when generation is so strong that the grid can't absorb it isn't saved in the form of hydrogen. Maybe only one hydrogen generation / electrical production plant for each of the few biggest fields..

After all, it's spare energy so even if hydrogen generation isn't economical per se, this could be, right? Does anyone know if whether is because of the electrode durability and hydrogen storage problems mentioned in the article or something else?

I thought that the navy was working on making jet fuel from seawater (cracking the CO2). What happened to all the hydrogen storage breakthroughs that happened awhile back, like storing hydrogen in tiny glass tube arrays that prevented explosive reactions.

I always wondered why the huge amount of energy generated in windmills that goes to waste when generation is so strong that the grid can't absorb it isn't saved in the form of hydrogen. Maybe only one hydrogen generation / electrical production plant for each of the few biggest fields..

After all, it's spare energy so even if hydrogen generation isn't economical per se, this could be, right? Does anyone know if whether is because of the electrode durability and hydrogen storage problems mentioned in the article or something else?

Most likely because that doesn't happen that often, so most of the time the infrastructure would be sitting their idle. It's just easier and more economical to build a gas-fired generator and use that to balance load.

It always seemed to me that the problem never was the source of the energy. There are many energy sources around, many of which are clean and renewable. The problem is long-term storage. In that regard, I was always intrigued by superconductors. Current will flow practically forever in a superconductor even after you remove the power source. You can tap into the stored current later on. If we could find a room temperature superconductor, we could create a nearly perfectly efficient power storage device. That is what would solve the energy problem.

>>The price of extraction is going up, the amount to be extracted is going down

This is simply not true where natural gas is concerned.

Well, the price of natural gas may have gone down, but given that there is a finite supply, the amount to be extracted will always decline unless no more is extracted. Eventually, prices will increase as supplies diminish.

If central banks would stop using gold as a fall-back currency, then we could free up tons of it to power the tech industry. But, since central banks love pumping gold back-n-forth between world economies with the booms and busts they create, gold will remain a scarce resource.

Could have swore they made discoveries recently to use very cheap, abundant agents in building these things... now we're back to expensive materials?

The researchers note that during the on-time, the current jumps to some high level, and then, instead of plateauing, it slowly increases by about a third. There is no explanation of this slower process.

I'm guessing that the current refers to the plasmons tunneling through the insulating barrier. If so, can't simply the light intensity from the lamp increasing over time explain the increased current flow? The intensity of many/most lamps increase over time (along with a shift in the spectrum) before they eventually stabilize.

A non-scientist here. What does it mean for me as a consumer waiting for his magical Free Energy device? Will I ever get it?

It uses sunlight to create fuel from water. It separates the hydrogen from the oxygen in water (H2O), so you can store the hydrogen and burn it to extract energy later.

Hydrogen fuel cells already exist. Some commercial uses are in cars (production cars not just concept cars), and largish boxes that power whole office buildings.

++ for your answer

Maybe I'm going too far off topic, but I'm fuzzy on how hydrogen is the miracle cure for our energy problems. I know eventually fossil fuels become so scarce that the cost to extract/refine them reaches a point that other fuel sources are more appealing. But, does the cost of extracting hydrogen currently beat the cost of extracting and refining fossil fuels? Also, wouldn't we net more energy by just combusting the hydrogen directly (in small, controlled amounts ... maybe like in an ICE that's been modified to use hydrogen instead of gas) instead of pumping it into a fuel cell?

I guess I'm interested in hearing some comparisons ... not only on which is cheaper (economics), but also on which is more energy efficient. I keep looking on the web, and all I see are companies that talk about how wonderful fuel cells are w/o giving solid stats about them, or tin-foil hatters talking about running cars on water and how big oil will never let us do it.

Inquiring minds want to know

edit:

To give a foundation of what I (think) I know so far, we get more energy from fossil fuels than we put into them by converting them to gas (am I right?) But, in extracting hydrogen and then using it in a fuel cell, does that create a net loss of energy?

To give a foundation of what I (think) I know so far, we get more energy from fossil fuels than we put into them by converting them to gas (am I right?) But, in extracting hydrogen and then using it in a fuel cell, does that create a net loss of energy?

The energy into producing the hydrocarbons(fossil fuels) has already been done by nature,so we're just making use of the result. Unless your going to start breaking thermodynamics, the whole closed system is going to be a net loss on useable energy. Hydrogen makes a great energy storage medium due to the few steps it takes to generate and it's energy density, also the byproduct of the fuel cells being simply water helps with the green issues in many ways. Storage and transportation are a couple of the issues that work against hydrogen being an easy transition right now though.

Also, wouldn't we net more energy by just combusting the hydrogen directly (in small, controlled amounts ... maybe like in an ICE that's been modified to use hydrogen instead of gas) instead of pumping it into a fuel cell?

A standard ICE has a maximum thermodynamic limit of 37% efficiency. A fuel cell can have efficiencies in the 40 to 60% range.

Tundro Walker wrote:

To give a foundation of what I (think) I know so far, we get more energy from fossil fuels than we put into them by converting them to gas (am I right?) But, in extracting hydrogen and then using it in a fuel cell, does that create a net loss of energy?

We can't generate more fossil fuels. They took 100's of thousands of years to form from dead organic life. That life used solar energy to build the hydrocarbons in the first place.

To give a foundation of what I (think) I know so far, we get more energy from fossil fuels than we put into them by converting them to gas (am I right?) But, in extracting hydrogen and then using it in a fuel cell, does that create a net loss of energy?

We can't generate more fossil fuels. They took 100's of thousands of years to form from dead organic life. That life used solar energy to build the hydrocarbons in the first place.

We can't economically generate more fossil fuels. The Navy (NRL) is working on making jet fuel(liquid hydrocardbons) from seawater. This is at a predicted cost of $3-6 a gallon once optimized and scaled. While this might eventually be useful for cars, this would be far to costly a replacement for primary energy sources such as coal.

To my taste, this piece is a bit rambling. It offers an excellent description of the chemistry utilized in the research, but little context.

By reading the abstract of the cited paper, I learned the investigators split water and produced hydrogen at efficiencies up to 20-fold higher than previously achieved. The efficiency increase was achieved by using visible spectrum solar radiation to power the reaction rather than UV spectrum.

It seems this research might contribute to a technical solution which will make a hydrogen economy economically viable. Apparently additional progress is needed to substitute a more common metal for the gold (and platinum?) used in the study.

We can't economically generate more fossil fuels. The Navy (NRL) is working on making jet fuel(liquid hydrocardbons) from seawater. This is at a predicted cost of $3-6 a gallon once optimized and scaled. While this might eventually be useful for cars, this would be far to costly a replacement for primary energy sources such as coal.

Expanding on that, in particular, they were looking to use excess electricity from the nuclear generators, seawater, and the carbon byproducts (unused food, human waste) generated by the meatbags (as Bender Bending Rodriguez refers to them as). This would allow them to make JP7 on aircraft carriers.

To give a foundation of what I (think) I know so far, we get more energy from fossil fuels than we put into them by converting them to gas (am I right?) But, in extracting hydrogen and then using it in a fuel cell, does that create a net loss of energy?

We can't generate more fossil fuels. They took 100's of thousands of years to form from dead organic life. That life used solar energy to build the hydrocarbons in the first place.

We can't economically generate more fossil fuels. The Navy (NRL) is working on making jet fuel(liquid hydrocardbons) from seawater. This is at a predicted cost of $3-6 a gallon once optimized and scaled. While this might eventually be useful for cars, this would be far to costly a replacement for primary energy sources such as coal.

Dumb, way-off-topic question follows ... how do we make synthetic oil? Is it merely oil that's been refined from a non-fossil fuel source?

Also, since fuel cells emit clean/pure H2O, could it be captured to use for human consumption, seeing as the limited amount of potable water on the planet at any given time seems to be a critical factor?

Also, since fuel cells emit clean/pure H2O, could it be captured to use for human consumption, seeing as the limited amount of potable water on the planet at any given time seems to be a critical factor?

*ponders a vehicle that gets heavier as your fuel tank become more empty.*

Also, since fuel cells emit clean/pure H2O, could it be captured to use for human consumption, seeing as the limited amount of potable water on the planet at any given time seems to be a critical factor?

There are more efficient ways to clean up water than separating it and burning it. So you would be wasting a lot of energy. That said, it is entirely possible. I have seen Jay Leno drinking water right from the tailpipe of a fuel cell vehicle. (That's not recommended of course lol.)

Also, since fuel cells emit clean/pure H2O, could it be captured to use for human consumption, seeing as the limited amount of potable water on the planet at any given time seems to be a critical factor?

There are more efficient ways to clean up water than separating it and burning it. So you would be wasting a lot of energy. That said, it is entirely possible. I have seen Jay Leno drinking water right from the tailpipe of a fuel cell vehicle. (That's not recommended of course lol.)

It always seemed to me that the problem never was the source of the energy. There are many energy sources around, many of which are clean and renewable. The problem is long-term storage. In that regard, I was always intrigued by superconductors. Current will flow practically forever in a superconductor even after you remove the power source. You can tap into the stored current later on. If we could find a room temperature superconductor, we could create a nearly perfectly efficient power storage device. That is what would solve the energy problem.

A big if also the superconductors have limits on the current density they will support so you can't store any amount in them.In most cases it is simply much more efficient to use nuclear than to use wind and solar. That way you avoid the issue of both fuel energy density as well as all the extra investment in to storage (some storage is good but with stable supply you need a lot less of it to balance the network).

If central banks would stop using gold as a fall-back currency, then we could free up tons of it to power the tech industry. But, since central banks love pumping gold back-n-forth between world economies with the booms and busts they create, gold will remain a scarce resource.

To give a foundation of what I (think) I know so far, we get more energy from fossil fuels than we put into them by converting them to gas (am I right?) But, in extracting hydrogen and then using it in a fuel cell, does that create a net loss of energy?

We can't generate more fossil fuels. They took 100's of thousands of years to form from dead organic life. That life used solar energy to build the hydrocarbons in the first place.

We can't economically generate more fossil fuels. The Navy (NRL) is working on making jet fuel(liquid hydrocardbons) from seawater. This is at a predicted cost of $3-6 a gallon once optimized and scaled. While this might eventually be useful for cars, this would be far to costly a replacement for primary energy sources such as coal.

Dumb, way-off-topic question follows ... how do we make synthetic oil? Is it merely oil that's been refined from a non-fossil fuel source?

Also, since fuel cells emit clean/pure H2O, could it be captured to use for human consumption, seeing as the limited amount of potable water on the planet at any given time seems to be a critical factor?

The Fischer-Tropsch process is used to create synthetic oils from carbon monoxide and hydrogen. The specific mix of these two gases is called "syngas" and it can be created from water and CO2 (called the water-gas shift reaction) or it can be a byproduct of other reactions. The reactions are difficult and dangerous on the industrial scale, which makes synthetic oil expensive.

There are other processes that can produce hydrocarbons from methane, hydrogen, and CO2, but they involve reacting these materials with oxygen at high temperature. Obviously, its very hard to do this without getting combustion and an explosion, so these processes haven't been scaled to industrial scales. With all the shale gas people have discovered, there is a huge push to further these reactions, and it is one of the areas I am working on right now. Unfortunately, there is very little I can tell you about that work, but you can read more here: http://www.carenafp7.eu/

To answer another question - hydrogen is interesting because it comes closer than most battery technologies to matching the energy density of hydrocarbon fuels, but it is still way, way off the standard of gasoline or diesel. Hydrogen fuel cells will operate at much lower temperatures than other similar devices, so it is also interesting for that reason. At this point, however, there is no storage solution that come even close to meeting the needs of safety, energy density, and portability. In my opinion, the hydrogen economy is either not coming, or is beyond our lifetimes. That said - the work described here is awesome!

Chris Lee / Chris writes for Ars Technica's science section. A physicist by day and science writer by night, he specializes in quantum physics and optics. He lives and works in Eindhoven, the Netherlands.